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Making Corrections For Noise

When a device under test (DUT) is measured with a spectrum analyzer, the resulting spectrum shown on the analyzer’s screen is actually a combination of the DUT’s input signal, the kTB noise, and the spectrum analyzer’s noise figure, NFSA. If the DUT’s input signal was disconnected, and the analyzer’s input port terminated in 50 Ω, the new spectrum would then be the combination of kTB and NFSA, or a trace on the screen of the spectrum analyzer’s noise floor.

Noise correction is a process by which the noise floor of the spectrum analyzer is measured, with the help of considerable averaging, and the noise floor trace is stored to a file called the Correction Trace. The DUT input signal is then connected to the spectrum analyzer and measured, with the results saved into a file called the Measurement Trace. Correction is applied when the Correction Trace is subtracted from the Measurement Trace and the results displayed in the Resultant Trace. The Resultant Trace is the spectrum of the DUT input signal with the excess noise removed, as shown by Eq. 6:

It should be noted that all values are converted from dBm (logarithmic) to mW (linear) before subtraction is performed, although the resulting trace is displayed in dBm. This process makes it easier to view low-level signals as well as make more accurate amplitude measurements, since errors from the contribution of the spectrum analyzer noise floor have been removed.

Figure 1 shows a relatively easy method for performing noise corrections with trace math. The noise floor of the spectrum analyzer is first averaged with the input terminated, with these results saved to trace 1. Then the DUT is connected and its signal captured and saved to trace 2. Trace math is then used for a power subtraction of the two traces, with the results saved to trace 3. The noise correction shows the most benefit when the input signal is close to the noise floor of the spectrum analyzer. Corrections have little or no effect on larger signals, which have a much lower contribution to noise.

1. These noise corrections were performed with the help of trace math.

The main issue with this approach is that the DUT must be disconnected and a 50-Ω load connected whenever a setting is changed. A method for measuring the Correction Trace without removing the DUT is to increase the input attenuation (say to 70 dB) to raise the spectrum analyzer noise floor far above the DUT input signal and then save this to the Correction Trace. The Correction Trace will now contain the components shown in Eq. 7:

Correction Trace = DUT Input Signal + kTB + NFSA + Atten (7)

If kTB + NFSA + Atten >> DUT Input Signal, it is possible to omit the DUT input level and state the Correction Trace according to Eq. 8:

Correction Trace = kTB + NFSA + Atten (8)

By subtracting the known attenuation from Eq. 8, it is possible to get back the original Correction Trace used in the manual method:

Correction Trace = kTB + NFSA (9)

The issue with this process is that the Correction Trace is valid only for the current settings of the spectrum analyzer. Changing settings such as center frequency, span, and RBW will invalidate the values stored in the Correction Trace. A better approach is to know the specific NFSA at all frequency points and then apply the Correction Trace for any setting.

The N9030A PXA signal analyzer from Agilent Technologies is calibrated during manufacturing to better understand its noise behavior, as part of its Noise Floor Extension feature (NFE; Fig. 2). At the time of manufacturing and calibration, the analyzer’s noise figure is measured across the entire frequency range of the instrument. This data is then stored within the memory of the instrument. When a user turns on the NFE feature on the analyzer, the analyzer calculates a Correction Trace based on the current setting of the instrument and the stored noise figure values. This eliminates any need for measuring the noise floor of the PXA, as was done in the manual procedure. This greatly simplifies the use of noise corrections and eliminates the excess time needed for measuring the noise floor of the instrument whenever a setting has changed.